1. Field of the Invention
The present invention generally relates to electronic mixers and more particularly to a surface-mount, even harmonic mixer exhibiting a high-input, third-order intercept point with low parasitic inductance and capacitance.
2. Description of the Prior Art
Second harmonic mixing using two anti-parallel Schottky diodes has been investigated by Cohn (M. Cohn, J. E. Degenford, B. A. Newman, Harmonic Mixing With An Antiparallel Diode Pair, IEEE S-MTT Int. Microwave Symposium Digest (1974) pp. 171-172) and Schneider (M. V. Schneider and W. W. Snell, Jr., Harmonically Pumped Stripline Down-Converter, IEEE Trans. Microwave Theory Tech., Vol. MTT-23, No. 3, March 1975, pp. 271-275). These investigations were extended by Neuf (D. Neuf, Even Harmonic Mixers Offer Unique Features for Millimeter Bands, Microwave System News, April 1982, pp. 103-119) to include four pairs of anti-parallel diodes in a bridge circuit. Four pairs of diodes enable the introduction of local oscillator (LO) energy with multi-octave radio frequency (RF) isolation from 2-18 GHz. Each of these references is incorporated herein by reference.
One way to understand the operation of an even harmonic mixer is to exploit the concept of a time varying radio frequency (RF) load. In this mode, also referred to as the reflection mode, incident radio frequency (RF) energy propagates toward the diode load, which is being driven by a local oscillator (LO) signal having a much greater amplitude. The action of the LO signal causes the diode load, and therefore the reflection coefficient presented to the RF signal, to vary as a function of time. Utilizing anti-parallel diode pairs enables conduction on both the positive and negative portions of the local oscillator (LO) voltage waveform. The resulting RF reflection coefficient varies at twice the rate of the LO signal.
Single- or multiple-pair, diode harmonic mixers achieve mixing action by reflecting RF energy from periodically changing diode impedances. Alternate diodes become forward-biased during the positive and negative cycles of the LO signal, which presents an “on” reflection coefficient of −1. During a short transition period, while there is insufficient LO voltage to turn the diodes on, an “off” reflection coefficient of +1 is presented. Hence, on and off ideal RF reflection coefficients of +/−1 occur twice for each cycle of the LO signal.
A phase difference of 180° between states must be maintained across the band of each of two different ports of a diode integrated circuit device. This requirement is easily satisfied at lower frequencies, but becomes exceedingly difficult to meet as frequency increases, due to parasitic inductance associated with the leads of the package.
An additional consequence of even harmonic mixing is, because of its single balanced nature, all mixing products appear across the diode bridge, which necessitates the use of a diplexer to separate the intermediate frequency (IF) output from the radio frequency (RF) input.
The reflected signal, which is a product of the load reflection coefficient and the incident signal, contains terms at RF, LO, 2×LO, RF−2×LO and RF+2×LO frequencies, as well as higher order terms. The term at RF−2×LO is referred to as the intermediate frequency (IF) and is filtered from the incident transmission line. The LO and RF signals are mutually isolated due to balance and symmetry, but the RF and IF signals exist in the same mode.
As a result, the reflection mode mixture is inherently single-balanced. The term at 2×LO is considered local oscillator (LO) leakage in a fundamental mixer. The symmetry of the even harmonic diode structure typically suppresses the 2×LO term an additional 20 dB compared to a fundamental mixer, making post filtering significantly less complex.
Even harmonic mixers are capable of achieving as low a conversion loss as fundamental mixers, and typically require less drive power from the LO signal for an equivalent single-balanced topology. This is advantageous since it results in a significant simplification of the local oscillator (LO) drive circuitry.
However, one disadvantage of even harmonic mixers is a reduced output 1 dB compression point when compared to fundamental mixers. The reduced output results from reduced input drive-level requirements of even harmonic mixers. A high-gain, low-noise amplifier (LNA), which is used in point-to-multipoint millimeter-wave radio system receivers to achieve noise figure requirements, forces the RF input power of the mixer to a relatively high level.
As a result, the third-order intercept point (IP3) performance of the mixer is critical to achieving overall system performance. The inability of conventional even harmonic mixers to meet IP3 requirements of the system has kept this type of mixer out of many communication receivers despite its many advantages. The third-order, input-intercept point is defined by U.S. Pat. No. 6,229,395 to Kay (column 5, lines 8-12), which is incorporated herein by reference, as a virtual measurement of the signal strength at which the third-order distortion energy power of the gain stage is as strong as the fundamental signal energy. IP3 is also used as an overall measure of linearity.
FIG. 1 shows a conventional, eight-diode ring mixer 10, which includes four anti-parallel diode-pairs 12, 14, 16, 18. The term “anti-parallel diode-pair” refers to a pair of diodes connected in parallel with opposing polarity. The mixer 10 requires two balum transformers 28, 38, but is still considered single-balanced. The addition of two transformers 28, 38 greatly adds to the complexity of the mixer 10 and limits its performance and frequency response due to parasitic inductances and capacitances.
As mentioned above, the mixer 10 includes four anti-parallel, diode-pairs 12, 14, 16, 18, each of which includes two diodes. The cathode of each diode in each anti-parallel pair of diodes is connected to the anode of the other diode in the pair resulting in a parallel connection of pairs of diodes wherein each anode is connected to a cathode and each cathode is connected to an anode. The anti-parallel diode pair 12 is connected to the anti-parallel diode pair 14 at node 20, the anti-parallel diode pair 14 is connected to the anti-parallel diode pair 16 at node 22, the anti-parallel diode pair 16 is connected to the anti-parallel diode pair 18 at node 24, and the anti-parallel diode pair 18 is connected to the anti-parallel diode pair 12 at node 26.
The first transformer 28 includes a first winding 30 connected in a series across nodes 24 and 20. A positive terminal of the first winding 30 is connected to node 24, a negative terminal of the first winding 30 is connected to node 20, and a center tap 32 of the first winding 30 is connected to ground. The first transformer 28 includes a second winding 34 connected in a series across a local oscillator (LO) input terminal 36 and ground.
The second transformer 38 includes a first winding 40, which has a positive terminal connected to node 26, a negative terminal connected to node 22, and a center tap connected to ground. The second transformer 38 also includes a second winding connected in a series across a radio frequency/intermediate frequency terminal 42 and ground. Additional details concerning the eight-diode mixer 10 are provided in J. L. Merenda, D. Neuf, and P. Piro, 4 to 40 GHZ Even Harmonic Schottky Mixer, IEEE MTT-S Digest (1988) pp. 695-698, which is incorporated herein by reference.
A conventional dual, anti-parallel diode pair single balanced mixer 44 is shown in FIG. 2. The mixer 44 is not able to achieve the input-intercept point exhibited by the conventional eight-diode mixer 10 shown in FIG. 1. The mixer 44 includes two anti-parallel pairs of diodes 46, 48 and a first transformer 50 having a first winding 52.
A positive terminal of the first winding 52 is connected to the anti-parallel diode pair 46 and a negative terminal of the first winding 52 is connected to the anti-parallel diode pair 48. The first transformer 50 includes a second winding 54 connected in series across a local oscillator (LO) input terminal 56 and ground.
A transformer shown as coils 58, 62, 64 is intended to represent an equivalent circuit (or lumped element representation) of a coplanar waveguide. The coplanar waveguide is electrically connected in series between the anti-parallel diode pairs 46, 48 and a radio frequency/intermediate frequency terminal 60.
Further examples of conventional mixers employing anti-parallel diode pairs are provided in U.S. Pat. No. 5,416,449 to Joshi; U.S. Pat. No. 5,771,449 to Blasing, et al.; U.S. Pat. No. 5,553,319 to Tanbakuchi; U.S. Pat. No. 5,787,126 to Itoh, et al.; and U.S. Pat. No. 5,995,819 to Yamaji, et al.; each of which are incorporated herein by reference. Examples of mixers using anti-parallel diode pairs are the HMC259 and HMC330, which are commercially available from Hittite Microwave Corporation, Chelmsford, Mass. 01824.
Even harmonic mixers are often used in point-to-multipoint millimeter wave radio systems, which will now be described. A variety of multichannel RF signal distribution systems are currently employed to deliver commercial broadcast television programming to residential customers. These RF transmission systems are often called “wireless cable” television systems, because they can provide multichannel entertainment programming identical to conventional cable television services, but without the cost and disruption incurred in installing video cable between the program provider's studio and each customer's residence.
United States electronic equipment suppliers have manufactured RF transmission systems to provide Multichannel Multi-Point Distribution Service (MMDS). These MMDS systems have been installed in major metropolitan areas and are used by the television entertainment industry to augment conventional television broadcasts by transmitting premium video programming to residential subscribers on a fee (pay-per-view) basis.
MMDS uses allocated spectrum at various frequencies in the 2.1 to 2.7 GHz band to transmit fourteen independent channels of video. The MMDS transmitters are installed at locations authorized by the United States Federal Communications Commission (FCC). Each of these transmitter locations has been selected so that it can broadcast into the surrounding service area without creating interference in the adjacent service areas.
In responding to the need for additional wireless, multi-point, television, distribution spectrum, that is, in addition to the authorized MMDS spectrum, the FCC issued an interim operating license in the 27.5 to 29.5 GHz band. The technology employed for use of this spectrum has been designated as LMDS and one implementation of an LMDS system is disclosed in U.S. Pat. No. 4,747,160 to Bossard, which is incorporated herein by reference.
Both LMDS and its predecessor MMDS broadcast multichannel television signals into specified “service areas”. Service areas (also referred to as “cells”) identify non-overlapping geographic regions that receive interference-free transmission from separate transmitter sites.
LMDS systems provide high-bandwidth, interactive services as the preferred wireless platform for enhancing and extending the current global broadband communications infrastructure. LMDS is distinct from other conventional copper cable, optical fiber, and low frequency wireless systems in its use of millimeter-wave frequencies for wireless distribution and cellular-like layouts for spectrum reuse and spectral efficiency.
The major advantages of millimeter wave distribution systems are the inherent broad transmission bandwidths that may be achieved and the opportunity to minimize the use and hence, the time and cost of implementing wired infrastructure. For example, 1 GHz of bandwidth centered at 28 GHz, has been allocated by the FCC for a one-way television service in the New York City metropolitan area.
A system formed in accordance with the teachings of U.S. Pat. No. 4,747,160 to Bossard, has been deployed under this allocation. The system uses essentially omni-directional cell-sites arranged in a center-excited cellular pattern to provide one-way television service to residential customers throughout the New York City metropolitan area using carriers centered around 28 GHz.
The signals transmitted by the cell-sites are received by high-gain/narrow-beam antenna/receiver units, which are normally located just inside or outside of a subscriber's window. The received signals are then down-converted and cabled to a set-top receiver and encryption unit that processes and conveys the video and audio signals to conventional, analog televisions systems.
Each cell has a channel assignment and polarization allocation that provides for the mitigation of co-channel and adjacent-channel interference making possible frequency reuse, and therefore improved spectral efficiency within a given coverage area. The center-excited coverage plan by the system disclosed in the Bossard reference is based upon geographically partitioned subscribers/receivers within given cells and assumes that each receiver assigned to a cell is serviced specifically by the one cell-site transmitter and omni-directional antenna that is geographically associated with the given cell in which the subscriber is located.
LMDS systems include a microcellular configuration including a large number of cells wherein each cell ranges from about 0.5 km2 to 2 km2 in size. The cells contain a base station serving many subscriber units and a large number of subscriber units are required to support an LMDS system. The subscriber units are sold to consumers and must be simply and inexpensively manufactured while maintaining an acceptable level of performance.
FIG. 3 shows a base station transceiver 66 and a subscriber unit transceiver 68. Generally, the base station 66 includes a base local oscillator 70, which provides a base local oscillator signal to a first input of a first base mixer 72. A modulated transmit signal is coupled to a second input of the first base mixer 72. The first base mixer 72 mixes the modulated transmit signal with the base local oscillator signal and frequency upconverts the modulated transmit signal.
The frequency-upconverted, modulated signal is transmitted to subscriber units through a first base antenna 74. The subscriber unit 68 receives the modulated signal from the base station 66 through a first subscriber antenna 76. The subscriber unit 68 also includes a subscriber local oscillator 78, which is coupled to a first subscriber mixer 80 for frequency downconverting the received modulated signal. The frequency downconverted modulated signal can then be demodulated.
Modulated signals are also transmitted from the subscriber unit 68 to the base station 66. The subscriber unit local oscillator 78 is coupled to a second subscriber mixer 82, which frequency upconverts a subscriber-modulated signal for transmission to the base station 66 through a second subscriber antenna 84.
The base station 66 receives the subscriber-modulated signal from the subscriber unit 68 through a second base antenna 86. The base local oscillator 70 is also coupled to a second base mixer 88, which frequency downconverts the received subscriber modulated signal. The frequency down-converted modulated signal can then be demodulated.
The subscriber unit 68 receives a high frequency (27.5-28.35 GHZ) digitally-modulated signal from the base station 66. The subscriber unit 68 frequency downconverts the received, high-frequency signal to an intermediate frequency (950-1800 MHZ) that a subscriber modem is able to demodulate.
The subscriber unit 68 also receives a low frequency (400-700 MHZ) digitally-modulated signal from the subscriber modem. The subscriber unit 68 frequency upconverts the low-frequency, modulated signal to a transmission frequency (31-31.3 GHZ).
Therefore, the availability for use in LMDS systems of an even harmonic mixer capable of achieving as low a conversion loss as fundamental mixers and also achieving as high an input intercept point as a fundamental mixer referenced to the same LO drive power level would be advantageous.